In an engine used for an automobile or the like, an exhaust turbocharger is widely used, in which a turbine is rotated by exhaust gas energy of the engine and a centrifugal compressor directly coupled to the turbine compresses intake air and supplies the engine with the intake air, to improve the output of the engine. Furthermore, turbochargers having a variable mechanism such as variable geometry (VG) turbochargers and waste-gate valve mounted (WG) turbochargers are becoming popular. By controlling the operation condition of a turbocharger with such a variable mechanism, the boost pressure is adjusted to a pressure suitable to the operation state of the engine, and the fuel consumption efficiency and drivability of the engine are improved. On the other hand, a turbocharger may enter an abnormal operation state called surging, depending on the operation condition. Surging leads to breakage of a turbocharger, and thus it is critical to perform an avoidance operation (avoidance control) as soon as possible to avoid surging, if surging is detected.
Generally, surging can be detected by fluctuation of pressure, flow rate, temperature, or the like. For instance, in Patent Documents 1 and 2, the operation point of the compressor in operation is detected from the pressure ratio of pressures at the inlet and the outlet of the compressor and the inflow amount (intake flow rate) of intake air into the compressor. Further, it is confirmed whether the detected operation point is positioned in the surge operation region where surging could occur defined on the compressor map, and an avoidance operation of surging is performed if the operation point is in the surge operation region. Furthermore, a specific technique to perform the avoidance operation of surging, as in Patent Document 1, includes setting the opening degree of operation devices, such as the opening degree of a compressor bypass valve, the nozzle opening degree of a VG (variable geometry nozzle mechanism), and the opening degree of a WG valve, to fully open. Some techniques do not fully open the opening degree of an operation device. For instance, Patent Document 2 discloses preventing occurrence of surging with a margin by setting a surge prevention line taking into account the responsiveness of the compressor bypass valve for a surge line which is a boundary between the surge operation region and the normal operation region, when opening the compressor bypass valve, and controlling the operation point with reference to the surge prevention line.